The invention relates to fused silica. More particularly, the invention relates to a method of reducing radiation-induced damage in fused silica articles, such as lenses. Even more particularly, the invention relates to a method of using radiation, such as infrared radiation, to reduce such radiation-induced damage in fused silica articles.
Fused silica is used in optical members such as lenses, prisms, filters, photomasks, reflectors, etalon plates, and windows. Many of these optical members are used in various apparatus employed in environments where they are exposed to ultraviolet light such as, for example, an excimer laser beam or some other ultraviolet laser beam, having a wavelength of about 360 nm or less. The optical members are incorporated into a variety of instruments, including lithographic laser exposure equipment for producing highly integrated circuits, laser fabrication equipment, medical equipment, nuclear fusion equipment, or some other apparatus which uses a high-power ultraviolet laser beam.
Laser technology has advanced into the short wavelength, high energy ultraviolet spectral region, the effect of which is an increase in the frequency (decrease in wavelength) of light produced by lasers. Of particular interest are short wavelength lasers operating in the UV, deep UV (DUV), and vacuum UV wavelength ranges. Such lasers include, but are not limited to, those operating at about 248 nm, 193 nm, 157 nm, and even shorter wavelengths.
Excimer laser systems are widely used in microlithography applications. The short wavelength of the excimer laser enables the preparation of circuits having decreased feature sizes by allowing for increased feature resolution and thus line densities in the manufacturing of integrated circuits and microchips, thus enabling the manufacture of circuits having decreased feature sizes.
A direct physical consequence of shorter radiation wavelengths (and higher frequencies) is higher photon energies. In optical systems such as those mentioned above, fused silica optics are exposed to high irradiation levels for prolonged periods of time. Such high radiation levels may result in the degradation of the optical properties of the optical members. Such laser induced degradation adversely affects the optical properties and performance of the fused silica optics by decreasing light transmission levels, discoloring the glass, altering the index of refraction, altering the density, and increasing absorption levels of the glass. Changes in density and index of refraction cause wavefront distortion and polarization-induced birefringence (also referred herein as “PIB”), which slowly increase over the course of exposure. Such wavefront distortion and PIB reduce the useful lifetime of the fused silica optics and thus limit the optical performance of the system.
One current approach to this problem is to identify silica compositions that are resistant to DUV radiation-induced damage. Here, manufacturing parameters, such as the concentration of water vapor utilized, are varied, or various dopants are added to the base silica composition to increase the resistance of the silica glass to radiation-induced damage. Although previous techniques have been effective, there is a continuing search for additional improvements that are directed at improving the resistance of fused silica to radiation-induced damage and mitigating such damage.
Another approach is to develop processes that reduce the laser-induced damage. Thermal treatment in the range of 125° C. to 400° C., as described by Piao et al. (“Thermal Annealing of Deep Ultraviolet (193 nm) Induced Compaction in Fused Silica,” J. Vac. Sci. Technol. vol. B16(6), 1998, p. 3419), for example, is known to reduce wavefront distortion after exposure to DUV light. However, the use of high temperatures precludes such heat treatments in complex lithography systems.
Presently, the ability to increase or improve the resistance of fused silica to DUV radiation-induced damage or repair such damage is limited by either composition or processing restraints. Therefore, what is needed is a method of repairing such radiation-induced damage and the wavefront distortion and polarization-induced birefringence resulting from such damage. What is also needed is a fused silica article, such as a lens, having reduced wavefront distortion and polarization-induced birefringence.